The present invention relates to a broadcasting signal detecting circuit in a broadcasting receiver, and particularly to a circuit for detecting the broadcasting signals of a SECAM system in accordance with the existence or absence of a color burst signal within a predetermined period in video cassette recorders or television receivers for PAL system.
European countries adopt either the phase-alternating line (PAL) system or the sequentielle a memoire (SECAM) system as their standard television broadcasting system.
Accordingly, television broadcasting receivers, e.g., a TV receiver, a video tape recorder, and a camcoder now on the market, have circuitry which enables the receiver to receive signals of both the above-mentioned systems. In such a double-broadcasting receiver, a circuit is required to distinguish whether the received TV signal is of the PAL system or the SECAM system.
A circuit for detecting SECAM signals is illustrated in FIG. 1. The circuit can be used, for example, in a PAL video tape recorder which can also record SECAM signals.
Referring to FIG. 1, a first amplifier 10 amplifies only a color burst signal from input color signals entered from an unshown signal input source by gating in accordance with a burst gate pulse BGP. That is, the BGP is input to the first amplifier 10 as a control input. The amplifier 10 only outputs the input color signal when the BGP at the control input is high.
Here, the gating is carried out because the signal entered from the signal input source, i.e., the SECAM or PAL signal, has a sub-modulated subcarrier at the burst flag.
The first amplifier 10 has a 4.5 MHz cut-off frequency. Filter 20 passes only those signals output from amplifier 10 that are within the cut-off frequency.
At this time, when the received signal is of a SECAM system, since the color burst frequency is alternately converted into 4.40625 MHz and 4.2 MHz every horizontal period, the output from filter 20 is amplitude-modulated as shown in FIG. 2A. Then, the amplitude-modulated signal is detected and flattened by a first detector 30, as illustrated in FIG. 2B.
Since the amplitude of the output from first detector 30 differs for each horizontal period, a 7.8 KHz signal corresponding to half of the horizontal frequency (1/2H) is amplified by a second amplifier 40. Then, when the amplified signal is again detected and flattened in a second detector 50, a DC voltage as shown in FIG. 2C can be obtained.
A comparator 60 outputs a logic "high" which results from comparing the output signal of second detector 50 shown in FIG. 2C with a reference voltage Vref.
On the other hand, in case of a PAL signal, since the subcarrier is constantly 4.4 MHz, and the amplitude is not modulated, a signal such as shown in FIG. 2D is output from filter 20.
The output signal (FIG. 2E) of first detector 30 which receives the output signal (FIG. 2D) of filter 20 has no 7.8 MHz component, but does have a horizontal frequency component.
Since the output signal of second amplifier 40 is small, the detected and flattened output from second detector 50 shown in FIG. 2F is also small. Thus, the final DC output voltage becomes a logic "low".
Therefore, comparator 60 outputs a logic "high" for SECAM signals, and a logic "low" for PAL signals. Thus, it is possible to determine whether a SECAM or a PAL signal has been received.
Also, Japanese patent publication No. sho 63-27187 discloses a signal discriminating circuit which determines whether a signal is of the PAL or SECAM systems. Here, using the difference between voltage levels which correspond to the burst signal frequency of each broadcasting system, the output voltage is fixed to be an identical DC voltage during the burst period, and to be a predetermined different voltage during periods besides the burst period.
However, the above-described conventional broadcasting signal detecting circuits have problems in that malfunctions may occur according to peculiarities the broadcasting area or component variations in the applied circuitry.